1. Field of the Invention
The invention relates to a photosensitive microcapsule employed in an image forming device for forming an image, and more particularly, to a photosensitive microcapsule that does not reduce the density of the image formed and does not reduce the sensitivity of the microcapsule to exposure to light of a predetermined wavelength.
2. Description of Related Art
Conventionally, a photosensitive microcapsule encapsulating therein image forming material and photosensitive material, and an image forming method using the photosensitive microcapsules have been proposed. The image forming material also comprises at least one of a dye precursor, a dye, and a pigment. The photosensitive material changes its mechanical strength by reacting to light.
There is a photosensitive recording medium disclosed in Japanese Laid-Open Patent Publication No. 61-275742. The photosensitive recording medium comprises photosensitive silver halide, reducing agent, polymerizable compound, and the image forming material comprising at least one of a dye precursor, a dye, and a pigment, which are coated on a base sheet. In the photosensitive recording medium, at least the polymerizable compound and the image forming material are encapsulated in one microcapsule. Moreover, there is a photosensitive pressure-sensitive recording sheet disclosed in Japanese Laid-Open Patent Publication No. 62-39844, which comprises a base sheet and a plurality of microcapsules coated thereon. Each microcapsule encapsulates image forming material comprising colored dye and pigment and light hardening resin. The image forming material and the hardening resin are dissolved or dispersed in an organic solvent.
Each microcapsule involves the photosensitive material and the image forming material which are dissolved or mixed with each other therein. When the microcapsule is exposed to light, having a predetermined wavelength, the involved photosensitive material changes the mechanical strength of the microcapsule by reacting to the light. According to the above-mentioned property of the photosensitive material, if these microcapsules are exposed selectively to the light, the mechanical strength of the microcapsules is selectively changed. Further, if each microcapsule of which the mechanical strength is selectively changed has a pressure applied, the microcapsules having low strength, that is, unchanged strength, are ruptured. The image forming material flows from the ruptured microcapsules. The released image forming material is absorbed into or adhered onto a receiving medium so that an image is formed on the receiving medium.
However, in the above-mentioned microcapsules there are problems. A first problem is that an image formed with the above-mentioned microcapsules has an uneven density or has low resolution because the image forming material is condensed, or forms globules, in each microcapsule, that is it is not uniformly dispersed therein. Thus, when the image is formed on the receiving medium, each picture element which is formed with the image forming material, is formed as large blobs rather than a uniform image. To solve this first problem, it is preferable that the image forming material exists in the microcapsule as very fine particles. That is, it is preferable that the image forming material and the photosensitive material be completely dissolved in each other. Alternatively, it is desired that the image forming material be dispersed finely in the photosensitive material. However, even if an arbitrary image forming material and an arbitrary photosensitive material are selected and heated or are mixed mechanically, it is difficult to make the image forming material and the photosensitive material be completely dissolved in each other. Similarly, it is difficult to have the image forming material dispersed finely in the photosensitive material. In other words, if a photosensitive material and an image forming material are selected to be soluble in each other, the image forming material and the photosensitive material will be completely dissolved with each other. Alternatively, if a photosensitive material having a property for easily dispersing the image forming material is selected, the image forming material is dispersed finely in the photosensitive material.
However, in practice, the photosensitive material employed in the microcapsule is selected based on its photosensitive sensitivity property and the photosensitive wavelength and the image forming material is selected based on its coloring property and its color reproducing property. Therefore, if the image forming material and the photosensitive material are selected as mentioned above, it is very difficult to obtain an image forming material and a photosensitive material that are completely dissolved in each other or those in which the image forming material is dispersed finely in the photosensitive material.
A second problem occurs when the photosensitive sensitivity of the conventional microcapsule is insufficient. There are many kinds of the photosensitive material having sufficient photosensitive sensitivity. However, when the photosensitive material is mixed with the image forming material in the microcapsule, the photosensitive sensitivity of the photosensitive material is remarkably reduced. Therefore, in order to form the image with the microcapsules, there is a need to apply a great amount of energy to the microcapsules, that is, to expose the microcapsules with a great amount of energy for a long time. Therefore, it is difficult to employ the microcapsules in a copier or a printer because copiers and printers have to form an image quickly. Although precise reasons why the photosensitive sensitivity of the microcapsule is insufficient cannot be stated, it is believed that the image forming material, which does not contribute to the hardening of the photosensitive material, disturbs that hardening by actions such as polymerization.